Main Building at Bletchley Park – photo by Matt WhitbyCredit: photo by Matt Whitby

Some secrets are hard to keep, especially when hundreds are in on it; yet the secret of Bletchley Park was kept for decades.  During World War 2, Bletchley Park was the location of the United Kingdom’s main decryption organization, the Government Code and Cypher School (GC&CS). This is where ciphers and codes were broken; codes and ciphers of several Axis countries.  The most important work was the deciphering of ciphers generated by the German Enigma and Lorenz machines. Bletchley Park also housed a message sending station and a secret radio intercept station; though the interception was soon moved to a location with better reception.  The high-level intelligence produced at Bletchley was codenamed Ultra.  

The Early History of Bletchley Park 

Bletchley Park is located fifty miles north-west of London.  In 1883 wealthy financier, Herbert Samuel Leon, bought over 300 acres of land beside the London and North-Western Railway line which passed through Bletchley, Buckinghamshire.  He developed sixty acres into a country estate and at the heart he built a mansion with a mixture of architectural styles.  

After Herbert and his wife Fanny died, the Park fell into the hands of Captain Hubert Faulkner, a property developer who had intentions of demolishing the buildings and selling the land.  However, that never happened.  In 1938 Hitler invaded Austria and Czechoslovakia and the threat of war was high.  The GC&CS was based in London and wanted a safer place to conduct business; someplace protected from enemy air attacks.  Bletchley Park was perfect as it sat at the junction of major road, rail and teleprinter connections to all parts oHut 6 at Bletchley Park  – photo by Matt WhitbyCredit: photo by Matt Whitbyf the country.  

The facility was commanded by Alastair Denniston and given the code name “Station X” as it was the tenth of a large number of locations acquired by MI6 for its operations. They built 18 wooden huts called Huts 1 through Huts 18; and brick buildings A through H.  Code breakers arrived in August 1939 after much preparation and trials runs.  The code breakers' cover was “Captain Ridley’s Shooting Party.” 

Breaking the Code During WW II 

The code breakers at Bletchley had their work cut out for them.  The Germans were using the Enigma machine and believed it to be unbreakable.  The complexity of the machine brought theEnigma Machine Used During WW II – photo courtesy of the CIACredit: photo courtesy of the CIA odds of breaking the codes to a staggering 150 million, million, million, to one.  However, the Poles broke the Enigma in 1932 when the Germans were conducting trials with the machine. During that time, the cypher altered only once every few months.  Although the war changed the frequency to at least once a day; the Poles had already passed on their knowledge to the British and the French.  This helped the code breakers make critical progress in working out the order in which the keys were attached to electrical circuits.  In turn, this knowledge enabled the code breakers to exploit a fundamental flaw in design in which no letter could ever be encrypted as itself.  In addition, errors in messages sent by German operators gave them clues to deciphering the codes.  

There were a total of 18 huts, but the Enigma decrypt teams worked in Huts 3, 6, 4 and 8.  The huts operated in pairs and were known only by their numbers for security reasons.  Code breakers who concentrated on the Army and Air Force cyphers were based in Hut 6 and supported by a team in Hut 3.  Hut 4 concentrated on naval intelligence and teamed with Hut 8 which decoded messages from the German Navy.  Supporting teams prepared the intelligence reports which were then forwarded to MI6, thus keeping the deciphering and the intelligence derived from it separate for security reasons.  The raw material came from “Y” Stations which comprised a web of wireless intercept stations throughout Britain and several overseas countries.  

Alan TuCode Breaker Machine at Bletchley Park – Photo courtesy of the Archives of the United KingdomCredit: Photo courtesy of the Archives of the United Kingdomring, a brilliant mathematician, helped speed up the code breaking process when he developed an idea originally proposed by Polish cryptanalysts. This invention was called the Bombe.  The Bombe was an electro-mechanical machine which greatly reduced the odds in breaking the daily-changing Enigma keys; which in turn, reduced the time required to break the codes.   

Ultra was significant in defeating the German U-boats as well as in the desert campaign in North Africa.  Because of the work at Bletchley Park, the Allies knew the location of all but two of the 58 German divisions on the Western front prior to the Normandy landings on D-Day. When the United States entered the war, a number of American cryptographers were posted to Bletchley Park and were eventually integrated into the Ultra structure.  

Recruitment for Bletchley Park 

Approximately 9,000 people, both military and civilians were working at Bletchley Park during the height of the code breaking efforts in January 1945.  During the war over 12,000 worked there at some point.  Women made up 80% of the workforce.  Initially, recruitment came from personal networking from Cambridge, Oxford and Aberdeen Universities.  Women who were recruited for clerical and administrative positions were similarly recruited.  

Cryptanalysts were recruited for their various intellects.  Selections made included linguists, crossword experts, great mathematicians, chess champions, and those with the ability to speak several languages with a high degree of proficiency.  They worked in three shifts, six days a week.  The shifts rotated and after three weeks they went off at 8 a.m. and came back at 4 p.m. which made the last workday sixteen hours long. They were allowed a thirty minute lunch break in the middle of each shift.  The work was tedious and required great concentration.  Workers were given one week leave four times per year.  

Security of the Code Breakers 

To keep security at a high level, in addition to keeping the deciphering and sending of intelligence reports separate, Bletchley Park staff were sworn to secrecy about their jobs.  Any improvements to the operating policies or procedures on the parts of the Germans could set back the deciphering process significantly.  One such setback occurred when the German Navy introduced the four-rotor Enigma used for communicating with U-boats.  The change stopped the ability to read this network from February to December in 1942. 

The slightest suspicion by the Axis powers of their cyphers being broken could lead to a change. Therefore, as part of the security at Bletchley Park, all staff were required to sign the Official Secrets Act of 1939.  In addition they were directed to never discuss their work outside their immediate section.  A May 1942 personal security form stated: 

  • Do not talk at meals ...
  • Do not talk in the transport ...
  • Do not talk travelling ...
  • Do not talk in the billet ...
  • Do not talk by your own fireside ...
  • Be careful even in your Hut ... 

Bletchley Park Today 

The directives to keep silent were well accepted. So much so, it wasn’t until the F.W. Winterbothams’ book, The Ultra Secret was published in 1974 people who worked at Bletchley Park finally started to reveal information about their wartime work.  Even today, some former employees of the secretive Bletchley Park still regard themselves bound to keep their silence. Code Breaker Decipher Machine – photo by Matt WhitbyCredit: photo by Matt Whitby

After the war, much of the equipment and blueprints were destroyed.  The site belonged to several owners including organizations that used the facility as a training center.  By 1991 the facility was practically empty and the buildings were close to being demolished for redevelopment.  February 1992 Milton Keynes Borough Council declared most of the Park a conservation area. The Bletchley Park Trust was formed to maintain the site.  

The Trust is volunteer-based and relies on public support to keep the Bletchley Park open.  Currently the site holds a museum dedicated to the code breakers.  The museum features many objects relating to the code breaking during WW II as well as numerous other exhibition centers.  Many of the buildings continue to be renovated back to their original condition.





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